In recent years, there is a demand for high recording density in hard disk drives (HDD) with a rapid growth of about 40% per year. In order to achieve such a high growth rate, it is necessary to write data to a magnetic recording medium with smaller track width and bit length. This leads to the requirement of further micro fabrication of the magnetic recording head. However, when the magnetic recording head is micro fabricated, the recording magnetic field generated from the magnetic recording head is also reduced. As a result, sufficient magnetic field may not be obtained to record data to the magnetic recording medium. Thus, attention is drawn to a technology that facilitate writing to the magnetic recording medium, by supplying an additional energy to the magnetic field generated by the magnetic recording head alone to achieve a desired level. The technology, called energy assisted recording, reduces the magnetic field necessary for the magnetization reversal by adding some energy to the medium, in order to write magnetic signals to the recording medium in which recording may not be possible in general. Thus, the technology makes it possible to increase the recording capability and reduce the recording area. There are two methods known as energy that is used for assisting such recording. One is a recording method using the heat generated by a laser beam (heat-assisted magnetic recording). The other is a recording method using the microwave generated by a high frequency oscillator (microwave-assisted magnetic recording).
In particular, the recording technology using microwave has just recently emerged and is expected to be one of the energy assisted recording. JP-A No. 243527/1994 discloses a technology for easy writing with a small recording magnetic field by irradiating a recording element of a magnetic head, or a magnetic recording medium, with a high frequency field, to locally reduce the coercive field strength of the medium by the Joule heating or eddy current heating.
J. G. Zhu and X. Zhu, ‘Microwave-assisted Magnetic Recording,’ The Magnetic Recording Conference (TMRC) 2007 Paper B6 (2007) discloses a technology (microwave-assisted magnetic recording technology) using a high frequency magnetic field. The technology is designed to generate microwaves by providing a field generation layer (FGL) in which the magnetization is rapidly rotated by a spin torque located adjacent to a main pole of a perpendicular magnetic head, in order to record information to a magnetic recording medium with a large magnetic anisotropy. Further, Y. Wang, et. al, “Media damping constant and performance characteristics in microwave-assisted magnetic recording with circular as field”, Journal of Applied Physics, vol. 105, pp 07B902-07B902-3 (2009) discloses a technology that is designed to provide a spin torque oscillator between the main pole of the magnetic recording head and a trailing shield, in order to effectively assist the magnetization reversal of the magnetic recording medium by changing the rotation direction of the high frequency magnetic field according to the recording magnetic field polar character.
According to the reports in these documents, it is found that the microwave-assisted magnetic recording can be combined with the existing perpendicular magnetic recording head, so that research and development of microwave-assisted magnetic recording has been accelerated due to its high feasibility.
The existing write head has a magnetic gap (hereinafter referred to as TS gap) between a main pole and a trailing shield. Normally, a current is not supplied to the main pole and the trailing shield, so that in general the material constituting the TS gap is an insulating material.
However, in the case of microwave-assisted magnetic recording, in general, a spin torque oscillator is provided between the main pole and the trailing shield. It is necessary to supply current to the spin torque oscillator to generate a high frequency magnetic field. In other words, it is necessary to supply current to the main pole and the trailing shield.
However, the configuration of the existing write head is different from the configuration of the microwave-assisted magnetic recording. For this reason, there are three important problems in the supply of the current.
The first problem is the variation of the resistance due to the anisotropic magneto-resistance (AMR) effect (hereinafter referred to as the AMR effect) or the eddy current. In the existing write head, writing is performed by supplying current (called recording current) to a coil located in the vicinity of the main pole, to excite magnetic film such as the main pole and the trailing shield. When the current is supplied to the spin torque oscillator to obtain assist by microwave irradiation, the direction of the magnetization in the magnetic film varies according to the increase and decrease of the recording current. At this time, when the main pole and the trailing shield are used as a line to supply the current to the spin torque oscillator, the relative angle between the direction of the magnetization in the magnetic film and the direction of the oscillator current is changed. As a result, the AMR effect occurs.
There is a risk that the resistance of the magnetic film material may vary due to the AMR effect. When the current is supplied with a fixed voltage to the spin torque oscillator in which the resistance varies due to the AMR, the current value flowing through the spin torque oscillator also varies, thus preventing stable oscillation of the spin torque oscillator. Further, when the direction of the magnetization in the magnetic film varies according to the increase and decrease of the recording current, the eddy current flows through the magnetic film so as to prevent the variation. This is particularly significant when the recording current is reversed. Because of this phenomenon, the oscillator current varies, thus preventing stable oscillation of the spin torque oscillator. The influence of the two phenomena is significant when the pattern of the main pole and the trailing shield is reduced to meet the demand for higher recording density. Thus, this is one of the considerable problems.
In order to solve the first problem, as described in JP-A No. 070541/2009, there is a method for providing a non-magnetic line between the spin torque oscillator and the main pole, and between the spin torque oscillator and the trailing shield, respectively, instead of using the main pole and the trailing shield as the line.
However, when the electrode layer is inserted in the TS gap as described above, the magnetic gap between the main pole and the trailing shield is increased. The magnetic field to be vertically applied to the lamination direction of the spin torque oscillator is reduced. As a result, it is difficult to increase the frequency of the generated high frequency magnetic field, and there is a risk that sufficient assist effect may not be obtained. Thus, the second problem is not to increase the TS gap, and preferably, to reduce the TS gap.
The third problem is to reduce the element resistance including the line of the spin torque oscillator. The main pole and the trailing shield, which sandwich the spin torque oscillator therebetween, are designed to be able to produce a strong magnetic field to record information to a recording medium. For this reason, it is difficult to reduce the resistance of the spin torque oscillator as the line by freely forming the shape and film thickness thereof. Further, when a magnetic material is used as a line, there is a risk that the line resistance is increased. This is because the resistance ratio of magnetic metals generally used as lines is higher than the resistance ratio of non-magnetic metals.
On the other hand, it is necessary to increase the spin torque effect in order to oscillate the spin torque oscillator. One method for obtaining a large spin torque effect is to supply current with a high current density. In this case, it is desirable that the line resistance is low, taking into account the long-term reliability and the influence of the resistance increase due to the heat generation.